大型低圧タービンローター用鋼のオーステナイト結晶粒の成長に及ぼす鍛造後の保持時間の影響

抄録

Austenite grains in the core of large scale turbine rotor forgings are refined by repeating annealing treatments afterforging in order to improve the ability of detection of flaw by ultra sonic wave. The treatments, however, require high cost and long time. The grain growth behavior of austenite during holding at high temperature following hot deformation was investigated using hot deformation simulator to achieve refinement in austenite grain by controlling hot forging.
Austenite grain markedly grew to about 4mm during holding for 5000s at 1250°C after a small amount of deformation of 5% at 1250°C. In the specimens in which extremely large austenite grains were formed, deformed austenite recrystallized statically by bulging mechanism and there were many grain boundaries with unstable triple point in recrystallized austenite before grain growth. On the other hand, only a few grain boundaries with unstable triple point were observed in samples deformed at more than 10%. Since a small strain is applied to forgings at the last stage of forging, it appears extreme grain growth occurs. There is a possibility that extreme grain growth can be prevented by accumulating strain to more than 8%, which can be achieved by forging with a period within 30s between deformations at a certain position.